1. Field of the Invention
This invention is in the field of non-destructive testing, particularly non-destructive testing methods employing magnetizable particles which may also have the characteristic of fluorescing under ultraviolet irradiation. The specific improvement of the present invention is directed to the application of a light, fluffy hydrophobic coating onto the surface of the workpiece prior to application of an aqueous suspension of magnetizable particles whereby the particles are retained in flaws on the magnetized workpiece and the water of the suspension is shed from the surface.
2. Description of the Prior Art
Magnetic particle inspection methods have been used for flaw detection for many decades. Basically, these nondestructive testing methods includes the step of supporting a magnetizable workpiece and passing a direct or alternating current through the piece in order to magnetize the same. Contemporaneously, magnetic particles either in dry form or in suspension in water or oil are applied to the surface. The stray magnetic fields produced at any surface flaws serve to attract the magnetic particles so that they are readily visible upon inspection.
More recently, the magnetic particles have been combined with fluorescent particles either in the form of a water or oil suspension so that the workpiece is inspected under filtered ultraviolet or black light to observe any concentration pattern of fluorescent particles caused by a surface discontinuity. Prior art patents referring to this type of inspection include Switzer U.S. Pat. No. 2,267,999 and Kazenas U.S. Pat. No. 2,936,287. These patents relate, respectively, to lacquer bonded and resin bonded fluorescent magnetic particles for use in magnetic particle inspection processes.
One of the more important areas in which magnetic particle inspection is used is that of inspecting steel billets in a continuous manner. Normally, the billet is inspected for longitudinal seams and, in the case of fluorescent particles, the inspection takes place under black light in a darkened inspection area.
One of the difficulties when using the fluorescent magnetic particles in aqueous suspension is the difficulty to fix the indications on the workpiece, since the surface area remains wet for a considerable period of time. Furthermore, the background of the workpiece, such as a steel billet, tends to be slightly fluorescent in itself so that tears and other faults are poorly indicated through the use of the fluorescent magnetic particles.